Tiltrotor aircraft typically include multiple propulsion assemblies that are positioned near the outboard ends of a wing. Each propulsion assembly may include an engine and transmission that provide torque and rotational energy to a drive shaft that rotates a proprotor assembly including a hub assembly and a plurality of proprotor blades. Typically, a pylon assembly, which includes the proprotor assembly, is rotatable relative to the wing such that the proprotor blades have a generally horizontal plane of rotation providing vertical lift for takeoff, hovering and landing, much like a conventional helicopter, and a generally vertical plane of rotation providing forward thrust for cruising in forward flight with the wing providing lift, much like a conventional propeller driven airplane. In addition, tiltrotor aircraft can be operated in configurations between the helicopter flight mode and the airplane flight mode, which may be referred to as conversion flight mode.
Physical structures have natural frequencies of vibration that can be excited by forces applied thereto as a result of operating parameters and/or environmental conditions. These frequencies are determined, at least in part, by the materials and geometrical dimensions of the structures. In the case of tiltrotor aircraft, certain structures having critical natural frequencies include the fuselage, the wing and various elements of the propulsion assemblies. An important environmental condition experienced by tiltrotor aircraft is forward airspeed, which may induce proprotor aeroelastic instability, such as proprotor whirl flutter, which may couple to the wing of a tiltrotor aircraft. In the event of such coupling, the wing can become unstable, leading to excessive vibration, flutter or structural failure. To prevent such coupling, most wing airframes are designed to be stiff and light. For example, the wing of a conventional tiltrotor aircraft may include a torque box that is structurally suited to absorb wing deflections and help ensure wing stability. Nonetheless, it has been found that forward airspeed-induced proprotor aeroelastic instability is a limiting factor relating to the maximum airspeed of tiltrotor aircraft in forward flight mode.
In current aircraft, the wing and fuselage are separate structures that, while physically linked, are not integrated with one another. In addition, wings are typically mounted to the fuselage without supporting structure designed to distribute wing loads across an enlarged area of the fuselage airframe, thus subjecting the area of the fuselage adjacent or directly underneath the wing to excessive loads. Further, such loads on the fuselage may be exacerbated by aeroelastic instability-induced wing deflections and can become particularly problematic if structurally weakening features, such as a door opening, are positioned underneath the wing.